This invention relates to angular rate sensors and more particularly to a vibrating cylinder gyroscope.
The vibrating cylinder gyroscope (VCG) found its origin in a scientific paper in the Proceedings of the Royal Society in London by G. H. Bryan in 1890, "On a Revolving Cylinder or Bell," Proc. Roy. Soc. London Vol. 47 (1890) pp. 101-111. This paper investigated the behavior of nodal points in a vibrating cylinder when rotated about its axis. Subsequently, much research and development has taken place on this type of rotation sensor, which has the potential advantage, when compared with conventional high-speed rotor gyroscopes, of being of much reduced mechanical complexity and hence cost and of increased ruggedness and life. A description of VCG operation is given in a paper by C. H. J. Fox and D. J. W. Hardie, "Vibratory Gyroscopic Sensors," DGON Symposium, Stuttgart, 1984. Basically, a thin-walled cylinder is made to oscillate radially at its lowest mode at its natural frequency. As will be discussed in more detail below, there are two nodal diameters for this simplest mode (n=2). The deflected form of the cylinder is basically elliptical and the vibration pattern is stationary when the cylinder is not rotated. When the cylinder is rotated about its axis, however, Coriolis forces act on the cylinder walls causing the vibration pattern to change its orientation in a direction opposite to that of the imparted rotation. If the cylinder rotates through an angle .phi., the pattern, with its nodal points, rotates through an angle of approximately 3/5 .phi.. This principle forms the basis of a vibrating cylinder rotation sensor. A rate integrating gyro results from measuring these nodal shifts.
Much work has been done on the vibrating cylinder gyroscope including work by Sperry, Kearfott and others. Two developments, the Hemispheric Resonator Gyro (HRG) by Delco and the Solid State Angular Rate Transducer (START) by GEC Avionics have reached a phase where the practicability of the principles has been demonstrated. A heightened interest in the VCG has now been created by the need for a small, low-cost and rugged rate sensor to be used in inertially guided projectiles. For these applications, the sensor must be capable of surviving linear accelerations up to 16,000 g's and rotational accelerations up to 247,000 rad/s.sup.2. The sensor must have a full scale range of .+-.500.degree./s, an overall accuracy, inclusive of threshold, resolution and hysteresis of 0.01.degree./s and a ready time of less than 0.05 seconds. The sensor should also be small, having a size not exceeding three cubic inches.
Vibrating cylinder gyros, though simple, have not found wide usage because they cannot easily be made to meet performance requirements. Important practical problems must be overcome particularly in relation to the characteristics of the cylinder. Even in a theoretically perfect and optimum ring with a wall thickness that can be ignored with respect to the ring diameter, some error will occur due to centrifugal terms when the natural frequency of the cylinder is too low with respect to the applied rotation rate of the cylinder. The method of supporting the cylinder to a reference frame, the homogeneity of its material, the damping characteristics of the material, the degree of symmetry and the degree of thermal equilibrium are factors that affect the node locations of the cylinder, and therefore, the response of the vibrating cylinder gyroscope. In addition, the apparatus for exciting the cylinder at its natural frequency and detecting the nodal points must be extremely symmetrical and stable. Test data published by Delco with an instrument made of fused quartz indicate drift errors as low as 0.01.degree./h when thermally modeled and 1.degree./h when unmodeled. The scale factor accuracy is better than 1 ppm. The initial data published by GEC Avionics with a resonator ring made of metal indicate drift errors of approximately 0.1.degree./s and a scale factor linearity of better than 0.25%. The dispersion in accuracy between the two known instruments is ascribed to the vastly different level of sophistication, complexity, and thus cost, by which the instruments are built. A conservative performance estimate of a low-cost VCG made of metal (which is much less expensive and more rugged than quartz) would be as follows:
Drift Error: 50.degree./h PA1 Scale Factor Accuracy: 0.1% of full scale PA1 Range: .+-.500.degree./s
An object of the present invention, therefore, is a vibrating cylinder gyroscope which is relatively accurate, low-cost, rugged and having a long life time.
Yet another object of the invention is a VCG having a freely vibrating cylinder free of any attachments or deposits and machined accurately to eliminate the need for balancing.